1. Field of the Invention
The present invention relates to a supercharged 4-cycle engine adapted to supply a fuel-air mixture through a crank chamber into a combustion chamber and compress the fuel-air mixture by utilizing a change in volume of the crank chamber in association with reciprocation of a piston.
2. Description of the Related Art
Conventionally known from Japanese Patent Laid-open Nos. 47-35516 and 61-200330 is a supercharged 4-cycle engine adapted to supply a fuel-air mixture through a crank chamber into a combustion chamber and compress the fuel-air mixture by utilizing a change in volume of the crank chamber in association with reciprocation of a piston. A typical structure of such a supercharged 4-cycle engine will be described with reference to FIGS. 2 to 4.
FIG. 2 shows a general structure of a single-cylinder supercharged 4-cycle engine. A cylinder 2 is mounted to an upper portion of a crank case 1. A cylinder head 3 is mounted to an upper portion of the cylinder 2. A crank shaft 5 is provided in a space (crank chamber) 4 of the crank case 1 so as to be rotatably supported to bearings (not shown). A piston 6 adapted to be slidably reciprocated in the cylinder 2 is connected through a connecting rod 7 to the crank shaft 5. A cam shaft 8 is provided in the crank chamber 4 so as to be rotatably supported to bearings (not shown). A crank gear 9 is press-fitted to the crank shaft 5 at one end thereof. A cam gear 10 is press-fitted to the cam shaft 8 at one end thereof. The crank gear 9 is in mesh with the cam gear 10. The number of teeth of the cam gear 10 is set to be twice the number of teeth of the crank gear 9. An intake cam 11 and an exhaust cam (not shown) for respectively driving an intake valve and an exhaust valve to be hereinafter described are fixed to the cam shaft 8.
A rocker cover 12 is mounted on an upper end of the cylinder head 3 to define a valve operating mechanism accommodating space 13. A valve operating mechanism 18a is accommodated in the space 13 surrounded by the rocker cover 12. An intake port 15 is formed in the cylinder head 3 so as to communicate with a combustion chamber 14 in the cylinder 2. An intake valve 16 is supported in the cylinder head 3 so as to open and close the intake port 15. Similarly, an exhaust port (not shown) is formed in the cylinder head 3 so as to communicate with the combustion chamber 14, and an exhaust valve (not shown) is supported in the cylinder head 3 so as to open and close the exhaust port. The valve operating mechanism 18a includes a spring 17 for biasing the intake valve 16 in a valve closing direction thereof, a spring (not shown) for biasing the exhaust valve in a valve closing direction thereof, and a rocker arm 18 for biasing the intake valve 16 in a valve opening direction thereof. An adjusting screw 19 is threadedly engaged with the cylinder head 3, and the rocker arm 18 is rockably mounted at its central portion to the adjusting screw 19 over the cylinder head 3. An upper end of the intake valve 16 is in contact with one end of the rocker arm 18. The other end of the rocker arm 18 is in contact with an upper end of a push rod 20. A tappet 21 is fixed to a lower end of the push rod 20, and is in contact with the intake cam 11. Although not shown, another valve operating mechanism for operating the exhaust valve is similarly constructed. A push rod passage 22 is formed around the push rod 22. The push rod passage 22 is communicated through a communication hole 23 with the crank chamber 4. Accordingly, the valve operating mechanism accommodating space 13 is in communication with the crank chamber 4.
A carburetor 24 for forming a fuel-air mixture is connected through an intake passage 25 to the crank chamber 4. A reed valve 26 is provided at a connecting portion between the intake passage 25 and the crank chamber 4 so as to permit suction only of the fuel-air mixture into the crank chamber 4. Further, the crank chamber 4 is connected through a supercharging chamber 27 to the intake port 15. A reed valve 28 is provided at a connecting portion between the crank chamber 4 and the supercharging chamber 27 so as to permit discharge only of the fuel-air mixture from the crank chamber 4 into the supercharging chamber 27.
In a compression stroke and an exhaust stroke of the engine, the piston 6 is lifted to increase a volume of the crank chamber 4 and reduce a pressure in the crank chamber 4. Accordingly, the reed valve 26 is opened as shown in FIG. 3, and the fuel-air mixture formed in the carburetor 24 is sucked through the intake passage 25 into the crank chamber 4. On the other hand, in an explosion stroke and an intake stroke of the engine, the piston 6 is lowered to decrease the volume of the crank chamber 4 and increase the pressure in the crank chamber 4. Accordingly, the reed valve 28 is opened as shown in FIG. 4, and the fuel-air mixture in the crank chamber 4 is discharged into the supercharging chamber 27. Accordingly, the fuel-air mixture in a compressed and high-pressure condition is stored in the supercharging chamber 27. This mixture having a high pressure is supplied through the intake port 15 into the combustion chamber 14 when the intake valve 16 is opened in the intake stroke.
The provision of the supercharging chamber 27 brings about an increase in quantity of the fuel-air mixture to be supplied into the combustion chamber 14 by 30-40% as compared with a normal aspiration type engine. Further, in order to reduce a fluctuation in supercharging pressure and effect stable supply of the fuel-air mixture, a volume of the supercharging chamber 27 is set to a value 10-20 times a displacement of the engine.
An output of such a supercharged engine can be improved by increasing a compression ratio of the fuel-air mixture in the crank chamber 4 to increase a supercharging efficiency. The compression ratio in the crank chamber 4 can be expressed as follows:
Compression ratio=(V.sub.D +V.sub.C)/V.sub.C =V.sub.D /V.sub.C +1
where V.sub.D represents a displacement of the engine, and V.sub.C represents a volume of the crank chamber 4 at a bottom dead center of the piston 6 plus a volume of the space communicated with the crank chamber 4 (i.e., the valve operating mechanism accommodating space 13 and the push rod passage 22). Accordingly, in order to increase the compression ratio, it is necessary to reduce the value of V.sub.C.
Regarding lubrication in the crank chamber 4, a lubricating oil is preliminarily mixed with a fuel, and such a mixed fuel containing the lubricating oil is atomized in the carburetor 24 to become a fuel-air mixture which is in turn sucked into the crank chamber 4, thereby effecting lubrication in the crank chamber 4. Further, the fuel-air mixture containing the lubricating oil in the crank chamber 4 is partially supplied through the communication hole 23 and the push rod passage 22 into the valve operating mechanism accommodating space 13, thereby effecting lubrication in the space 13 accommodating the valve operating mechanism 18a (especially, a contact portion between the rocker arm 18 and the push rod 20, and a contact portion between the rocker arm 18 and the intake valve 16).
As apparent from the above expression relating to the compression ratio, it is optimum to reduce the volume of the space 13 constituting the space communicated with the crank chamber 4, in order to increase the compression ratio of the fuel-air mixture. However, since the rocker arm 18 adapted to be rocked is accommodated in the space 13, it is difficult to greatly reduce the volume of the space 13. Accordingly, it is difficult to expect the improvement in engine output by increasing the compression ratio.
Further, as mentioned previously, it is necessary to set the volume of the supercharging chamber 27 to a value 10-20 times the displacement, in order to reduce a fluctuation in supercharging pressure. Accordingly, the supercharging chamber 27 is constructed in an expanded condition as shown in FIG. 2, and it is therefore difficult to make the engine compact.
Further, the valve operating mechanism 18a is one of the most necessary portions to be lubricated, and the lubricating oil for lubricating the valve operating mechanism 18a is supplied together with the fuel-air mixture through the communication hole 23 and the push rod passage 22 into the space 13. However, replacement of the fuel-air mixture between the crank chamber 4 and the space 13 through the communication hole 23 and the push rod passage 22 cannot be well performed. Accordingly, the lubrication of the valve operating mechanism 18a cannot be sufficiently effected.